mach 3 computer

I recently bot a mini grizzly converted to cnc and xylotex controller, and have a computer question. how critical is the separate video card for running mach3? I have a couple machines that are like 2+ ghz desktop units (Dell and Gateway) with 0.768 to 1 gig ram, but they DONT have separate video cards. Should they work? I also have some lesser units - 450 Mhz to 0.9 ghz. the 450 actually has a separate video and parallel printer cable, and the 0.9 ghz is an AMD Athlon with separate video card. What to do, pls? Thanks Chuck.